Transformer pressure relief devices



Oct. 29, 1957 E. c. WILLIAMS TRANSFORMER RREssuRE RELIEF 'DEVIcEs Filed June 16, 1952 INVENTOR. EUNYN C. WILLIAMS A T TORNE Y ilnited States Patent TRANSFORMER PRESSURE RELIEF DEVHIES Elwyn C. Williams, St. Louis Park, Minn., assignor to Minneapolis Honeywell Regulator Company, Minneapolis, Minn, a corporation of Delaware Application June 16, 1952, Serial No. 293,331

2 Claims. (Cl. 174--15) This invention pertains to transformers and more particularly to transformers hermetically enclosed within a housing or casing and provides for a positive and safe means for releasing and relieving excessive fluid pressures Within the transformer housing. The invention comprises the combination of a pressure relief plug secured in a transformer housing. The plug normally seals off the housing but responds to a predetermined differential of fluid pressure between the inside and the outside of the housing and relieves the pressure differential by allowing the passage of fluids, but of no other material, to flow through an aperture in the plug.

It is an object of the invention to provide a pressure relief device for a hermetically sealed transformer that will allow for the releasing of fiuid pressures within the transformer housing at a pressure below that at which the transformer case would fracture or be exploded.

it is a further object of this invention to provide a pressure relief device that safely releases fluids from a transformer housing.

Another object is to provide a pressure relief device for a transformer housing that prevents granular insulation material within the housing from being expelled to the outside of the housing.

A further object of the invention is to provide a pressure relief plug that can be removed from a transformer housing to allow for the insertion of insulation material I into the housing.

Another object of the invention is to provide a pressure relief plug for a transformer that is protected from injury from external sources.

A further object is to provide a pressure relief plug for a transformer that can be easily and conveniently replaced without disrupting the interior of the transformer housing. Still another object of the invention is to provide a pressure relief plug for a transformer that is reliable and does not deteriorate with age, contact with heat, humidity,

fungus, and the like.

These and other objects and advantages will become more readily apparent from the following description and the accompanying illustrative drawings.

In the industry supplying electrical apparatus for many applications and fields, but most especially for the field of aviation where the electrical equipment is subjected to a wide range of temperatures, external pressures, and other climatic conditions such as humidity, fungus, and the like, the practice has been to hermetically seal and pro- W tect thereby, as far as practical, all pieces of apparatus.

In the case of transformers, the procedure that is often used is to hermetically enclose the transformer in a housing with an insulation medium dispersed in the void spaces between the coil, core, and the housing. The insulation used may take one of several forms, and the housings or casings for the apparatus are usually made out of plastic material such as Bakelite or out of a metal such as steel or aluminum, a plastic material having been found desirable since it is strong, light, and can be shaped into a variety of forms.

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Under normal circumstances no trouble is experienced with the transformers as above described. However, transformers will sometimes overheat due to a fault therein or due to an overload or a short circuit on associated circuit components. The overheating is caused by the increased current due to the fault or short circuit flowing through the windings and reacting therewith in the well known manner to produce heat that must be dissipated by the apparatus. Under very adverse conditions the fault current may be of a relatively large magnitude and consequently the temperature rise of the apparatus may be rapid and of a large magnitude. This high temperature in turn reacts with the insulation material used so as to produce or generate gases, and as above mentioned, the cases are hermetically sealed so that the gases so generated cannot escape without some sort of mechanical failure occurring. Under the above mentioned adverse conditions with the accompanying high fault current and the subsequent high temperature rise and generation of gases, generally the mechanical component to fail is the housing or casing of the transformer. The housing may develop cracks and apertures which allow the gases and insulation material to escape, or the housing may violently explode releasing the pressures therein. When the former occurs, the insulation material and the gases will be spewed forth through the cracks and apertures, and when the latter occurs, it is obvious that the surrounding area will be bombarded with the pieces of the housing, hot gases, and hot insulation material. In either of the situations, at best, the surrounding area would be contaminated and it is conceivable that personnel and electrical and mechanical apparatus in the area would be adversely affected.

Much effort has been devoted to solving the problem of exploding transformer cases and the subsequent spewing forth of the hot gases and insulation materials. While no insulation material has been found to be unaffected by the high temperature rise so as not to generate gases, it has been found that some insulation materials are far superior to others. In general, it has been found that wax-type insulation materials and some casting resins are unsatisfactory in that at high temperatures they become fluid, and subsequent to the rupturing or fracturing of the transformer housing hot wax and dense obnoxious smoke is issued forth. One type of insulation material that has been found to be very satisfactory is the granular type of insulation. In particular, magnesium oxide, M g0, has been found to perform very well, it being a dry granular substance, and at the temperatures to which it is exposed in transformers, retains its granular structure, and further emits no objectionable smoke or odor when heated.

While the use of magnesium oxide overcame the problem of obnoxious smoke and hot melted insulations from being spewed forth from the ruptured transformer housings, it did not eliminate the fundamental source of the trouble, that is, the fact that expansion of materials inside the transformer housing cause the transformer housing to explode or rupture in an uncontrolled fashion.

It is with this problem in mind that I have conceived a device that normally plugs and seals the casing or housing of the transformer and prevents the flow of fluids, both gaseous and liquid, from the casing, but when the internal pressure of the casing reaches a predetermined abnormal value, that value being below the point where the casing would explode or rupture, the plug allows the fluids to escape safely and at the same time retains the relatively solid granular insulation material within the casing.

Reference may now be had to the drawings wherein is represented one embodiment of the invention. Figure l is a view in section of a transformer protected by a prescomprises two molded cup-shaped members, an upper 1 housing member 14 and a lower housing member 15. Housing members 14 and 15 are designed so as to overlap one another and to fit tightly together at their point of junction represented by shoulder 16 on member 15. A

resin-type adhesive, not shown, is used to make the junction between members 14 and 15 perfectly air-tight. The leads 17 and 18 for the primary and secondary windings respectively are brought out through the base of lower member 15 through bushings 20 which are positioned in apertures 21 in lower member 15 provided for that purpose. Solder and resin adhesives are used to insure an effective bond between primary and secondary leads 17 and 18 and the bushings 20 and between the bushings 20 and lower housing member 15 respectively so as not to nullify the hermetic seal of the transformer. Molded into lower member 15 of the transformer housing are stud members 22 which provide a means to fasten the transformer to a support, not shown.

Secured in an aperture in the upper member 14 of the transformer housing is a pressure relief plug 23 shown in greater detail in Figures 2, 3, and 4.

The procedure in assembling the transformer 10 is as follows. The core sections 11 and primary and secondary windings 12 and 13 are assembled as a unit in the well-known manner, the assembly being positioned inside of lower housing member 15. The primary and secondary leads 17 and 18 are brought out through bushings 29, the bushings 20 having previously been inserted through apertures 21 in lower housing member and bonded thereto by resin adhesive. bonded to the bushings by the use of solder. The upper member 14 of the transformer housing minus the pressure relief plug 23 is then fastened to lower housing member 15, the two housing members being securely bonded together at junction 16, a resin adhesive being used to give a mechanically strong and hermetic seal between the two members. A granular insulation material 24 such as magnesium oxide is then admitted into the inside of the transformer housing through aperture 25 in the upper housing member 14 in which is normally placed pressure relief plug 23, the insulation material being admitted in suflicient quantities so as to fill completely all the void spaces inside of the housing members 14 and 15. Pressure relief plug 23 is then secured in aperture 25 of member 14 and the transformer is then ready for operation.

Referring to Figures 2, 3, and 4, the pressure relief plug 23 is secured in aperture 25 of upper housing member 14 by means of cooperating threads 26, a tapered or pipe thread arrangement being shown. A circular boss or shoulder portion 27 of upper housing member 14 in the proximity of pressure relief plug 23 adds additional mechanical strength to the housing member. The use of threads 26 to fasten plug 23 to upper housing member 14 has been found to be the most satisfactory method of securing plug 23 although any suitable means of fastening such as a pressed fit arrangement or a retaining ring arrangement could be used under certain circumstances. The pressure relief plug 23 may be made out of a variety of materials; one that has worked very satisfactorily being the same type of plastic material from which the transformer housing is made.

The pressure relief plug 23 is circular in cross-section and is characterized by having an aperture therethrough of circular cross-section. The aperture comprises three The leads 17 and 18 are distinct portions, an inner portion 30 adjacent to the inside of the transformer housing, an intermediate portion 31, and an outer portion 32 adjacent to the exterior of the transformer housing. Inner portion 30 has the greatest diametrical dimension while outer portion 32 has the smallest diametrical dimension. The portions of plug 23 defining the junctions between the inner portion 30 and the intermediate portion 31, and the intermediate portion 31 and the outer portion 32 are shoulders 33 and 34 respectively. Placed in the inner portion 31 of the annular aperture and abutting against the circular shoulder portion 34 is a. circular filter-like mesh or screen or grating 35. Grating 35 may comprise a variety of ma terials, but one that has been found to perform very satisfactorily is metallic screening since it is light-weight, strong, and is easily worked. Placed in the inner portion 30 of the annular aperture and abutting against the circular shoulder portion 33 is a circular fluid impervious frangible wall or diaphragm 36, sealed against the circular shoulder 33 by cement or other suitable means, not shown. As will be discussed later, a metallic foil of aluminum has been found particularly suitable for the diaphragm 36. The diametrical dimensions of grating 35 and diaphragm 36 are such that they substantially fill the intermediate portion 31 and the inner portion 30 of the annular aperture respectively. Screen 35 is secured about its perimeter to circular shoulder 34 by cement or other suitable means, not shown, carev being taken so as not to allow any excess cement to clog the pores of the screen. A tool engaging slot 37 is provided in the outer surface part of plug 23 to facilitate the insertion of plug 23 into the housing member 14.

Operation Under normal operation of the electrical transformer 10, there exists a certain differential of pressure between the interior and the exterior of the housing 14 and 15. This is due in part to the fact that the transformer, while it is being operated, dissipates a certain amount of heat that in turn reacts with the contents inside the housing so as to somewhat raise the pressure therein. Also, if the transformer was used in an airplane, there would be an effective increase in pressure inside the transformer housing during the time that the aircraft was in the air due to the fact that the pressure on the outside of the housing had decreased. These types of pressure rises inside the transformer housing are expected and compensated for in the design of the housing and the pressure relief plug. However, if the transformer or its associated components should develop a fault or short circuit so that the current flowing through the primary and secondary windings 12' and 13 should appreciably increase beyond the normal value, a rapid and large temperature rise of the transformer will occur, which willreact with the insulation material to cause exceedingly high fluid pressures to be developed inside the housing. Since fluid pressures are transmitted to all parts of a container, the high fluid pressures developed by the high temperature rise will be transmitted to all parts of the interior of the transformer housing and will mechanically stress all parts of the housing including the portion in which is inserted the pressure relief plug 23. The thickness of the diaphragm 36 is carefully determined so that it will withstand the normal differential of pressure between the interior and the exterior of the transformer housing members but will rupture or fracture upon the attainment of a predetermined abnormal pressure differential caused by a fault or short circuit on'the transformer or its associated circuit components.

The fracture of the diaphragm 36 allows the abnormal differential in pressure to be equalized by the passage of gases through the annular aperture through the plug 23. When this occurs, acertain amount of the granular insulation material 24 dispersed inside the transformer housing will be tended to be carried along with the gases through the aperture and out into the surrounding area. The screen 35 serves the purpose of allowing the gases to escape subsequent to the rupturing of the frangible diaphragm 36 but retains the relatively solid granular insulation material 24 within the housing, the pores or openings in the screen being selected with that last-named purpose in mind.

It will be appreciated that the thickness of the frangible diaphragm 36 is the most critical dimension in the design of the plug 23. The thinner that frangible diaphragm 36 is, the lower will be the abnormal differential in pressure at which plug 23 will release. The material that frangible diaphragm 36 is composed of may be of a variety of materials that are capable of responding to pressure as required by the subject invention. One material that has Worked very satisfactorily is sheet aluminum, it being easily worked as to contour and thickness and is virtually unaffected by the passage of time, humidity, fungus, heat, and the like. However, even though aluminum is preferred, it is to be understood that the invention contemplates that the frangible diaphragm 36 may be made out of a wide variety of normally fluid impervious substances that would fracture or rupture upon the attainment of a suflicient pressure differential across the material.

Operation of the pressure relief plug 23 has shown that the frangible diaphragm 36 will be stretched or bowed out somewhat towards the outside of the transformer housing 14 by the differential of fluid pressure prior to the rupturing thereof. For this reason, it is obvious that suflicient clearance should be allowed between the frangible diaphragm 36 and the screen or grating 35 so as to prevent the screen or grating 35 from mechanically supporting the diaphragm 36 and thus impairing the operation thereof.

Another advantage of placing the screen or grating 35 between the exterior of the housing 14 and the frangible diaphragm 36 is that the frangible diaphragm 36, being rather delicate, is protected from being injured by objects that might be inserted into the annular aperture of the plug from the outside.

While I have shown and described a specific embodiment of my invention, other modifications will be readily apparent to those skilled in the art. I do not, therefore, desire the invention to be limited to the specific arrangements shown and described. For example, the shape and size of the aperture through the plug 23 may be varied in many different ways. It is intended that the appended claims will cover all modifications within the scope of the invention.

I claim as my invention:

1. In combination, an electrical transformer hermetically sealed within a hollow breakable housing, a finely divided insulation substance dispersed within said housing and about said transformer, and a pressure relief plug removably secured in an opening in said housing, said plug having an aperture therethrough, a first recess in said aperture, and a second recess in said aperture, :1 first closure spaced in said aperture and abutting said first recess, and a second closure spaced in said aperture and abutting said second recess, said first closure comprising a fluid impervious wall responsive to an abnormal differential of fluid pressure on opposite sides of said wall so as to allow the flow of fluid through said aperture and said wall to equalize said differential of fluid pressure, and said second closure comprising means having openings therein of a dimension less than the dimension or the particle of the granular insulation so as to permit the passage therethrough of fluids but do not permit the passage therethrough of said insulation substance.

2. In combination, a housing, an electrical transformer hermetically enclosed by said housing, a granular insulating substance disposed in said housing and about said transformer, and a pressure responsive pressure relief device comprising a plug adapted to be secured in an opening through said housing, said plug having an inner surface end and an outer surface end, said inner surface end being exposed to fluid pressures internal of said housing, said outer surface end being exposed to fluid pressures external to said housing, said plug having an aperture therethrough of circular cross-section, said aperture comprising an inner portion of relatively large diameter adjacent to said inner surface end of said plug, an outer portion of relatively small diameter adjacent to said outer surface and said plug, and a central portion spaced intermediate said inner and outer portions, said central portion having a diameter intermediate that of said inner and outer portions, the division between said inner and said central portions defining a first shoulder in said plug, the division between said central and outer portions defining a second shoulder in said plug, a fluid impervious circular diaphragm substantially the same diameter as said inner portion of said aperture positioned and secured to said first shoulder, and a rigid circular screen substantially the same diameter as said central portion of said aperture positioned against and secured to said second shoulder, said diaphragm rupturing upon a predetermined difierential between the internal and ex ternal pressures of said housing, said differential of pressures being equalized by the passage of fluids through said ruptured diaphragm and said screen, said screen having apertures therein of a smaller dimension than the dimensions of the particles of the granular insulation so as to offer no appreciable resistance to the passage of said fluid but tending to retain said granular insulating substance within said housing.

References Cited in the file of this patent UNITED STATES PATENTS 540,419 Burlingame June 4, 1895 833,694 Parker Oct. 16, 1906 1,476,138 Bingay Dec. 4, 1923 1,579,141 Pierce Mar. 30, 1926 1,599,245 Pornin Sept. 7, 1926 1,751,939 Meissner Mar. 25, 1930 2,234,042 Deeley Mar. 4, 1941 2,694,503 Young et al. Nov. 16, 1954 FOREIGN PATENTS 216,036 Great Britain May 22, 1924 229,312 Great Britain Feb. 11, 1925 482,518 Great Britain Mar. 30, 1938 563,418 Great Britain Aug. 14, 1944 

